The present invention relates generally to the field of marine seismic surveys and particularly to the field of deployment and retrieval of seismic sensors on the ocean bottom by autonomous underwater vehicles.
It is known that aggregates of solid minerals, e.g., manganese nodules, have been found on many areas of the deep ocean bottoms and other undersea floors. Underwater photography and television, and ocean bottom sampling techniques such as mechanical or suction dredging, have shown that manganese nodules are scattered in various concentrations at many different places on the deep sea floors. Moreover, spots where at least some manganese nodules have been found have been charted on maps of the ocean floors. Proposals have been made to mine the ocean floor to obtain commercially worthwhile amounts of minerals such as manganese nodules. In order to most efficiently apply undersea mining methods and equipment, it is important to identify and delineate undersea floor areas having a relatively large proportion of the floor, e.g., 25 or 50% or more of the floor area, covered with desired minerals such as manganese nodules.
U.S. Pat. No. 4,075,599 discloses a process and apparatus for underwater geophysical exploration to prepare surveys of undersea floor areas having solid minerals, such as manganese nodules, dispersed at sea floor surfaces transmits and perceives special acoustic vibrations providing information useful for identifying and delineating sea floor areas where desirably large amounts of solid minerals are present.
In the seismic surveying of submerged geophysical formations for gas and petroleum deposits, it is often desirable to gather wide angle reflection and refraction data which typically constitutes the bulk of the information obtained. Gathering seismic data for oil and gas exploration requires a greater separation of seismic wave sources and detectors than can be achieved with the co-located acoustic wave sources and sensors commonly used to gather monostatic reflection surveys for mineral resources.
U.S. Pat. No. 4,387,450 discloses a marine seismic data acquisition system whereby data is gathered by a single vessel beyond the range of a conventional towed seismic sensor cable through the use of expendable sensors and hard-wire transmission cables. The surface vessel tows a submerged platform adapted to carry several inexpensive seismic sensors and very small diameter multiconductor cables which are controllably released from the platform by appropriate equipment on the towing vessel. Signals generated by each sensor are transmitted back to the towed, submerged platform through the small diameter cable connected to each sensor and are transmitted by other means from the platform to the towing vessel for retransmission, recording and/or display. Each sensor transmits a signal back to the platform until its small diameter cable is completely deployed at which time the cable breaks and is abandoned together with the sensor. Very small diameter marine cables, which are commercially available in lengths of 20,000 feet and more are used to practice the invention, are coupled with an inexpensive hydrophone and preamplifier to allow the generation and reception of wide angle reflection and short range refraction seismic signals by a single vessel.
AUV""s have been designed to spool out fiber optic cable under ice caps in the ocean, however, these AUVs and cables are not neutrally buoyant, and require complex dynamic buoyancy adjustment mechanisms to compensate and balance the buoyancy of the AUV as it deploys cable. Such an AUV is discussed by J. Ferguson et al, in Theseus AUVxe2x80x94Two Record Breaking Missions, Sea Technology, pp. 65-70, February 1999. Moreover, the prior AUV do not retrieve the cables for redeployment. As cable leaves the AUV, weight is lost. To prevent this from affecting trim and buoyancy, the loss in weight is counteracted by an automatic buoyancy compensation system. Surrounding each cable spool is a toroidal hard ballast tank that is filled with water as the cable is dispensed from its spool. This keeps the buoyancy of each spool assembly near neutral. Metallic tabs at the end of each cable spool signal the vehicle control computer as each pack is emptied. This buoyancy compensation system is complex and adds weight and required size to the AUV.
The typical systems, discussed above, do not retrieve the hydrophones and cables for redeployment and reuse. This practice of abandoning the deployed hydrophones and cables after one use is expensive. Such abandonment requires stocking of multiple sets of hydrophones and cables for seismic coverage requiring more than one deployment of a hydrophone and cable system. Moreover, these conventional systems do not monitor deployed hydrophones so that inoperable hydrophones may be unknowingly deployed. Thus, an inoperable hydrophone would not be discovered until after a seismic data acquisition run. In such a case, new hydrophones would have to be deployed to replace the inoperable hydrophone and the seismic data acquisition run repeated because of inoperable hydrophones having been unknowingly deployed or deploying operable hydrophones in an inoperable position. Conventional systems have relied on passive coupling of hydrophones to the ocean bottom. These conventional systems rely on the combined negative buoyancy of the cable and hydrophones to sink to the ocean bottom and lie thereon. The hydrophones and cable are passively coupled to the ocean bottom by virtue of having come to rest thereon. Such passive coupling can cause suboptimal data due to the hydrophones not being well coupled to the ocean bottom and thus receiving less signal information from the ocean bottom.
Some passive systems have added weight to the cable and hydrophones to increase negative buoyancy intending to improve passive coupling, however, the additional weight complicates deployment and retrieval. Heavier cables and hydrophones can also decrease passive coupling because the heavier weight cable are stiffer to handle. Stiffer cables are less flexible and thus less likely to conform to the ocean bottom and more likely to form kinks on the bottom. The stiffer cables can actually decrease passive coupling and may require repetition of a seismic data acquisition pass due to misplacement of hydrophones which do not actually contact the ocean floor. Such repetition of seismic data acquisition is extremely expensive.
Conventional seismic data collection at the ocean bottom is thus problematic, costly and cumbersome. Conventional seismic data collection techniques require special vessels to deploy and retrieve heavy cables and equipment. Moreover, such deployment techniques physically distress the cable as it is being deployed and retrieved. Thus there is a need for a simplified, cost effective solution with quality control monitoring of the hydrophones and cables as they are deployed. There is also a need for a method and apparatus for deployment and retrieval of ocean bottom cable sensors from an autonomous underwater vehicle. There is also a need of a neutrally buoyant AUV and cable with active coupling of cable to the ocean bottom.
The heavy weight of conventional systems has made operations in water more than a few hundred meters in depth extremely slow, expensive and very hard on equipment. There is a need for a system capable of operating in thousands of meters of water.
The present invention overcomes the problems of the prior art discussed above. The present invention provides an autonomous underwater vehicle (AUV) that deploys and retrieves an ocean bottom seismic system comprising cables and seismic sensors. The AUV can separate from the deployed seismic system and return later to resume recording of seismic data. The present invention also monitors the operational status of the hydrophones and sensors during deployment so that inoperable hydrophones/sensors may be replaced while AUV is in position at the inoperable hydrophone location and prior to the seismic data acquisition. The AUV and cable/sensors are neutrally buoyant to reduce the size of the vehicle and eliminate the need for an apparatus which adjusts the dynamic buoyancy of the AUV for changes in buoyancy caused by dispensing negative or positively buoyant cables and sensors. The present invention attaches the neutrally buoyant cable to the ocean floor by means of a fastener thereby actively coupling the sensors to the ocean floor. The preferred cable and sensor comprises a uniform diameter incorporating sensors having a diameter less than or equal to the diameter of the cable in which they are housed.